Molecular dynamics of poly(N-isopropylacrylamide) in protic and aprotic solvents studied by dielectric relaxation spectroscopy.

We report the experimental results of dielectric relaxation spectroscopy for the systems of poly(N-isopropylacrylamide) [PNiPAM] in various solvents in the frequency range of 40 kHz to 20 GHz at the solution temperature of 25.0 °C. The solvents used in this study were protic solvents (water, methanol, ethanol, and 1-propanol) and aprotic solvents (acetone, methyl ethyl ketone, and dimethyl sulfoxide). Two relaxation processes were observed at frequencies of approximately 1 MHz and 10 GHz in all the solutions. The origins of the two relaxation processes are considered to be the reorientation of dipoles of the PNiPAM chains at middle frequencies (m-process) and that of solvent molecules at higher frequencies (h-process). For the PNiPAM solutions composed of protic solvents except for 1-propanol, the relaxation time of the h-process increased with increasing PNiPAM concentration, whereas that of the h-process for the 1-propanol decreased with increasing PNiPAM concentration. In contrast, the relaxation times of the h-process for the aprotic solvents were independent of the density of hydrogen-bonding sites. For the m-process, which is attributed to the local chain motion of PNiPAM, the extrapolated relaxation time to zero polymer concentration τ(m0) was scaled by the solvent viscosity for all the protic solvents, whereas for the aprotic solvents τ(m0) showed no correlation with the solvent viscosity. The dynamics of polymer chains and solvent molecules in their solution state are clarified in terms of cooperative motion, which is associated with the interactions through hydrogen bonding at the molecular level.